Mobile Circuit Switched (CS) services based on Global System for Mobile Communications (GSM) and Wideband Code Division Multiple Access (WCDMA) radio access are used world-wide. They allow a user to obtain telecommunication services with a single user subscription in most countries of the world. The number of CS subscribers is growing rapidly, boosted by the roll-out of mobile CS services in countries with high populations such as India and China. One reason the number of subscribers is still growing rapidly is the evolution of the Mobile Switching Centre (MSC) architecture into a softswitch solution, which allows the use of a Packet Switched (PS) transport infrastructure for mobile CS services.
A 3GPP work item, “Evolved UTRA and UTRAN”, defines a Long-Term Evolution (LTE), designed to improve efficiency, lower costs and improve services for 3GPP-based access technology. LTE will use Orthogonal Frequency-Division Multiplexing (OFDM) radio technology in the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) for the uplink, allowing at least 100 Mbps peak data rate for downlink data rate and 50 Mbps for uplink data rate. LTE radio can operate in different frequency bands and is therefore very flexible for deployment in different regions of the world, where different frequency bands might be licensed.
In addition to the Radio Access Network (RAN) standardization, a 3GPP System Architecture Evolution (SAE) work item is being to develop an evolved core network for LTE radio access networks. This new core network is also called Evolved Packet Core (EPC). The nodes and interfaces of the SAE core network and LTE radio access networks are illustrated schematically in FIG. 1. The SAE core network (SAE CN) is made up of core nodes, which may be further split into Control Plane (Mobility Management Entity, MME) nodes 1 and User Plane Gateway (Serving Gateway and PDN Gateway) nodes 2. In this application, the term Access Gateway (AGW) is used to depict both the Serving Gateway and the PDN Gateway nodes and functions. In the terminology currently used, AGW contains both User Plane Entity (UPE) and Inter-Access Anchor (IASA) functionality. The MME 1 is connected to an E-UTRAN Node B (eNodeB) 3 via a S1-MME interface, and the AGW 2 (i.e. the Serving Gateway) is connected to an eNodeB 3 via an S1-U interface.
LTE and SAE only support PS data transport, and so all services must be supported via a PS domain. However, existing GSM (GPRS) and WCDMA each provide for both PS and CS access and services, and so for telephony services to be deployed over LTE radio access, an IMS-based service engine is required. Solutions to allow LTE/SAE access to CS domain services normally available via GSM and WCDMA radio accesses are referred to as “CS over LTE/SAE”, or briefly just “CS over LTE” (CSoLTE) solutions. The basic architecture for these solutions is illustrated schematically in FIG. 2. It will be appreciated that references to “Circuit Switched services” throughout this document are intended to refer to the type of services traditionally available in the CS domain, however they are implemented.
The Packet MSC (PMSC) 4 may serve both traditional GSM and UMTS RANs 5 and the new CS over LTE based solutions 6. The PMSC 4 contains two logical functions referred to as a Packet CS Controller (PCSC) 7 and an Interworking Unit (IWU) 8, which are further illustrated in FIG. 3.
The communication between a terminal (MS) 10 accessing a network and the PMSC 4 is based on the standard Gi interface which is also called as a SGi interface in the SAE terminology. This means that all direct communication between the terminal 10 and the PCSC 7 and the IWU 8 in the PMSC 4 is based on IP protocols. The terminal 10 is visible and reachable using an IP-address via an Access Gateway (AGW) 2. This communication is via two different interfaces, U8c for the control plane and U8u for the user plane. The PCSC 7 has also an Rx interface to the Policy Control and Charging Rules Function (PCRF) for allocation of LTE/SAE bearers.
Different solutions for providing CSoLTE service have been identified. One solution is referred to as “CS Fallback”. In CS Fallback, the terminal performs SAE Mobility Management (MM) procedures towards a Mobility Management Entity (MME) 1 while using LTE access. The MME 1 registers the terminal in a Mobile Switching Centre Server (MSC-S) 9 for CS based services. When a mobile terminating call or other transaction request resulting in a page for CS services is received in the MSC-S 9 it is forwarded to the terminal as a CS Paging request via the MME 1 and then the terminal performs fallback to a GSM or UMTS RAN and responds to the CS Paging Request. This means that the terminal selects a cell in a GMS RAN (GERAN) or in a UMTS RAN (UTRAN) based on information received e.g. as part of the SAE MM procedures. A similar process is used for Mobile originated CS services, and when these are triggered and the terminal is using LTE access, it will fallback to a GSM or UMTS RAN and trigger the initiation of the CS service there.
Another solution is referred to as “CS over LTE Integrated” (CSoLTE-I). In this solution the same SAE MM and paging procedures as for “CS Fallback” are used, but instead of performing fallback to a GMS or UMTS RAN, the terminal performs all the CS services over the LTE access network. CS services (also called Connection Management, CM, procedures) are transported over IP-based protocols between a PMSC 4 and the terminal using the LTE access network and SAE nodes such as the AGW 2.
A further solution is referred to as “CS over LTE Decoupled” (CSoLTE-D). In this solution both MM and CM procedures are transported using IP-based protocols directly between the PMSC 4 and the terminal using the LTE radio access network and SAE user plane nodes such as the AGW 2.
The CSoLTE control plane protocol architecture between the terminal (MS) 10 and the PMSC 4 (i.e. the U8c interface) is illustrated schematically in FIG. 4. Interposed between the two are the eNodeB and the AGW. This architecture is based on IP protocols (IP, TCP, UDP) and an additional tunnelling protocol named as U8-Circuit Switched Resources (U8-CSR) which may for example be based on Generic Access Network (GAN) tunnelling protocols. This protocol carries the Mobility Management (MM) and all the protocol layers above MM transparently between the terminal and the PMSC 4. The U8c interface applies only for the CSoLTE-I and CSoLTE-D solutions.
The CSoLTE user plane protocols between the terminal and the PMSC 4 (i.e. the U8u interface) are illustrated in FIG. 5. eNodeB 3 and AGW 2 are arranged between the two. This architecture is based on IP protocols (IP, UDP, RTP) that are used to transmit the necessary voice and data communication (e.g. AMR coded voice) between the terminal 10 and the PMSC 4. The U8u interface is only used in the CSoLTE-I and CSoLTE-D solutions.
FIG. 6 illustrates current working assumptions for the fallback mechanism in the “CS Fallback” solution described above. The terminal (UE) 10 performs the fallback (i.e. selection of GSM or UMTS RAN and cell) based on information received during a Location Area/Tracking Area Update.
FIG. 6 also illustrates the signalling used to handle paging in the CS Fallback solution. A CS Paging Request is used to set up a signalling connection between the terminal 10 and the network (MSC/VLR 11). This signalling connection can then be used for different purposes, e.g. establishment of a mobile terminated call or delivery of a mobile terminated SMS from the network to the terminal. In a circuit-switched part of the GSM network, the location of the terminal 10 is maintained on Location Area (LA) level when the terminal is in so called idle mode and the current Location Area Identifier (LAI) is maintained at a Mobile Switching Centre 11 (MSC) or Visitor Location Register (VLR), and the terminal is then paged on that Location Area. When the MME 1 and eNodeB 3 receive the CS paging request, it is unconditionally forwarded to the terminal 10. Similarly, when the terminal 10 receives the CS paging request, it unconditionally tries to respond to it. This means that the terminal leaves the current LTE cell and performs fallback to a GSM or UMTS RAN and responds to the CS Paging Request in the GSM or UMTS RAN. The fallback to GSM or UMTS RAN means that the terminal selects a cell in a GSM RAN (GERAN) or in a UMTS RAN (UTRAN) based on information received e.g. as part of the SAE MM procedures.
A problem with the signalling illustrated in FIG. 6, is that the terminal 10 may already be using PS bearer services, for example VoIP, Mobile TV or some other real-time streaming services when the CS paging request is received. It is not possible for the MME 1 or eNodeB 3 to know if they should prioritise these current PS-bearers or the CS page request, since they are not service aware. This can be problematic if the PS bearers or services are considered to be important either for the operator or for the end user.
Currently it does not exist any function in the terminal 10 which allows it or the user to reject the CS paging request and to prioritise the current PS-bearers and services. This is, again, problematic if the PS bearers or services are considered by the end user to be important.